An example of such a sort of prior art cordless telephone systems is shown in FIG. 6. In the drawing, the system comprises a connection unit 10 which is connected to a subscriber line 2 of an exchange 1 in a central office and which includes a hybrid circuit 11 performing two-wire/four-wire conversion, a transmitter 12, a receiver 13, a control circuit 14 and an identification information memory 15. The system also comprises a radio telephone set 20 which is connected through a radio communication circuit to the connection unit 10 and which includes a transmitter 22, a receiver 23, a control circuit 24, an identification information memory 25, a key switch unit 26, a handset receiver 28 and a handset transmitter 28.
In such an arrangement, when an operator of the radio telephone set 20 depresses a calling key in the key switch unit 26 to made a call, information indicative of the call request is transferred to the control circuit 24. The circuit 24, when receiving the information, drives the transmitter 22 to transmit, in the form of radio waves on a control channel, a calling signal which comprises a bit synchronization signal, a frame synchronization signal and subsequently an identification code (ID code) unique to the radio telephone set 20, as shown in FIG. 7 (a).
The connection unit 10, when receiving the identification code of the radio telephone set 20 which forms a pair with the unit 10 on the control channel, transmits, from the transmitter 12 in the form of radio waves on the control channel, a calling answer signal which comprises a bit synchronization signal, a frame synchronization signal, and subsequently a speech channel assignment code and the identification code of the telephone 20, as shown in FIG. 7(b).
When the radio telephone set 20 receives the calling answer signal at the receiver 23, the control circuit 24 switches the speech channel to the one assigned by the received answer signal. The connection unit 10 itself, on the other hand, switches the speech channel to the one assigned by its own. And after the connection unit 10 has confirmed that the switched channel is idle, the unit establishes a DC closed loop between the hybrid circuit 11 and the exchange 1, modulates a dial tone transmitted through the DC closed loop from the exchange 1 and then transmits the modulated dial tone from the transmitter 12 to the radio telephone set 20. On the side of the telephone 20, when the receiver 23 receives this dial tone, the tone is sounded at the handset receiver 27.
At this stage, operator's dial-number dialling operation on the side of the radio telephone set 20 causes the dial number to be transmitted through the radio communication circuit and the connection unit 10 toward the exchange 1. The transmission completion of the dial number causes the connection exchanging operation of the exchange 1, thus calling the party subscriber line. As a result, if the party subscriber answers the call, then the system is put in the speech mode.
When the operator of the radio telephone set 20 wants to terminate his conversation, he returns the calling key of the telephone to the initial position to put the telephone in an on-hook mode. This causes the on-hook signal to be transmitted to the connection unit 10. When a predetermined constant time has elapsed after the transmission of the on-hook signal, the radio telephone set 20 also transmits a speech end signal to the connection unit 10. The unit 10, when receiving the speech end signal, `opens` the DC loop between the exchange 1 and the unit to release the subscriber line 2.
When the connection unit 10 receives a call from the subscriber line 2 in an await mode, the control circuit 14 of the unit 10 detects a calling signal received from the exchange 1 to transmit through the control channel of the transmitter 12 a call indicator signal which comprises the identification code of the radio telephone set 20 attached to a speech channel number beforehand checked and stored during the await mode operation.
The radio telephone set 20, when receiving the call indicator signal through the control channel, sets the channel of the receiver 23 to a speech channel assigned by the call indicator signal. When having confirmed that the assigned channel is idle, the telephone 20 transmits "channel switching completion signal" to the connection unit 10. The unit 10 itself also switches the speech channel to the one assigned by its own and when receiving the channel switching completion signal from the radio telephone 20, the unit 10 recognizes the establishment of the radio communication circuit and transmits a bell ringing signal to the radio telephone 20 in synchronism with the calling signal sent from the exchange 1.
The radio telephone set 20 generates a ringing tone from a sounder 30 in response to the reception of the bell ringing signal to call the user. When the user answers, it by picking up the handset of the telephone to provide an off-hook operation to the key switch unit, the off-hook signal is sent to the connection unit 10. The unit 10, when receiving this off-hook signal, establishes the DC closed loop between the exchange 1 and the unit to allow a speech between the subscriber line 2 and the radio telephone 20.
In the aforementioned calling and call indicator operations, the identification code of the radio telephone set 20 is previously stored in the identification information memory 25 of a ROM. When it is desired to call a subscriber line, the contents or ID code of the memory 25 is read out and transmitted to the connection unit 10. The unit 10, when receiving the ID code, compares it with the identification code previously stored in the identification information memory 15 to determine whether or not that the radio telephone 20 having issued the call forms a pair with its own for interconnection with the subscriber line.
When it is desired to make such a party-line cordless telephone system that a plurality of the radio telephone sets 20 are disposed in the service area of the connection unit 10 to allow the extension-call speech between the radio telephone sets as well as the outside-line speech between the radio telephones and the subscriber line, it is necessary to allocate individually unique identification codes to the respective radio telephone sets for the purpose of achieving the inter-call operation of the respective radio telephones. At the same time, the connection unit must store all the identification codes allocated to the radio telephone sets in the system.
When the unique identification codes are allocated to the respective telephone sets in this way, however, this involves such a problem that the connection unit must compare the received identification code with all the identification codes previously stored therein, which results in that a time necessary for transmitting a calling answer signal to the associated radio telephone becomes long. In addition, since the storage contents of the identification information memory must be modified according to an increase or decrease in the number of installed radio telephone sets, there occurs a problem that the management of the identification codes is troublesome and it is impossible to flexibly cope with the increase or decrease of the radio telephones.
Further, since a calling signal is issued with addition of only the identification code in the prior art, it is impossible to determine whether the calling signal is to be sent to the outside line or to the extension-call telephone, so that there is a possibility that, even in the case of the extension-call, a DC closed loop is undesirably established between the exchange and the connection unit.
Where a unique identification code is allocated to each of the plurality of radio telephone sets connected through a radio circuit to a connection unit connected to a telephone line, the connection unit must store a plurality of identification codes. Thus, the number of identification codes becomes several times larger than the number of the connection units. This configuration causes a problem in that as the number of bits in the identification codes increases the storage capacity necessary increases and transmission time of identification codes is increased. Furthermore, if there are 10,000 identification codes and each of the single party-line cordless telephone system is capable of having four of the radio telephone sets installed, the total number of such party line telephone systems which could be manufactured using a single set of identification codes would be 2,500. When it is required to install more than four radio telephone sets in each single party-line telephone system, the total number of such party line telephone systems is further reduced.